Synthesis and Photovoltaic Properties of a D- A Copolymer Based on the 2,3-Di( 5-hexylthiophen-2-yl) quinoxaline Acceptor Unit

A new D-A copolymer (PBDT-DTQx) based on the 2,3-di(5-hexylthiophen-2-yl)quinoxaline acceptor unit and a bithienyl-substituted benzodithiophene (BDT) donor unit is designed and synthesized for application as the donor material in polymer solar cells (PSCs). The polymer film shows a broad absorption band covering the wavelength range from 300 to 720 nm and a low highest occupied molecular orbital (HOMO) energy level at -5.35 eV. A device based on PBDT-DTQx:PC70BM ([6,6]-phenyl-C71-butyric acid methyl ester) (1:2.5, w/w) with chloronaphthalene as a solvent additive displays a power conversion efficiency (PCE) of 3.15%. With methanol treatment, the PCE of the PSCs is further improved to 3.90% with a significant increase of the short-circuit current density, J(sc), from 10.10 mA cm(-2) for the device without the methanol treatment to 11.71 mA cm(-2) for the device with the methanol treatment.